Apple mail suddenly stopped loading messages bodies and hugs over 100% CPU, weird activity window
Apple Mail stopped few days ago to download all new messages content. Headers are read in, but not bodies of the messages. I noticed it unfortunately with all Apple Support Communities messages, but it seems to affect other mails.
Mail is stuck to report a weird Activity (few messages only and no progress!). See picture below.
Clicking on Maibox->rebuild or synchronize does not seemingly do anything.
Mail CPU usage is continuously slightly over 100%
I run Mail from the same user account on a MacBook Pro 2015 (running Catalina 10.15.7 as well) and did not encounter any problem. So this problem happened only on my iMac late 2013
Catalina 10.15.7
Mail 13.4 (A single iCloud user registered, other users disabled temporarily)
iMac late 2013
Any idea what should I do? Br
iMac 27″, macOS 10.15